Theory and Modelling of Negative Skin Friction on a Pile in Soil-Article

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The study argues that negative skin friction (NSF)—downward shear along a pile caused by soil settling relative to it—can develop in essentially all piles, not only where major settlement triggers such as new fill, groundwater lowering, or earthquakes are present. Long-term monitoring has recorded NSF even in unloaded piles and where ground settlement was only a few millimetres.<br /><br />The authors propose that very small relative movements can mobilise pile–soil side resistance. Potential causes include soil disturbance and consolidation associated with pile installation, consolidation as installation-induced pore pressures dissipate, and settlement under sustained loads applied to the pile head. Because piles are much stiffer than surrounding soil, a neutral plane forms where pile and soil move together. Above it, soil moves downward relative to the pile and NSF increases the pile’s compressive force; below it, positive side resistance transfers load back to the soil.<br /><br />A FLAC3D model of a hypothetical pile confirmed that a small imposed soil movement was sufficient to produce this pattern. With no head load, compressive force rose from zero at the pile head to a maximum at the neutral plane, then declined as positive side resistance developed below. The model’s behaviour was consistent with field observations.<br /><br />NSF changes how piles respond to loads. The maximum axial force may occur below the pile head, and permanent-load stiffness depends on the ground below the neutral plane. Transient-load stiffness may be greater than conventional loading tests suggest, because temporary reversal of side resistance can occur with little pile-head movement. The authors therefore recommend accounting for NSF in structural and geotechnical design, even when substantial ground settlement is not expected.

Keywords

negative skin friction

pile settlement

neutral plane

pile-soil interaction

compressive force

side resistance

ground settlement

FLAC3D model

pile design

geotechnical engineering